Conventionally, a liquid crystal display device has been used in various electronic devices such as a television set, a laptop PC (Personal Computer), a desktop PC, a PDA (Personal Digital Assistant: a mobile terminal), and a mobile phone. This is because the liquid crystal display device has the following advantages: (i) it is thinner and lighter than a CRT (Cathode Ray Tube)-based display and (ii) it can be driven at a low voltage, and (iii) it can realize lower power consumption.
A liquid crystal display device in which TFT (Thin Film Transistor) elements are used realizes a high display quality since all pixels are driven via respective TFT elements.
Meanwhile, it is rapidly becoming popular to display a moving picture by use of a liquid crystal display device in a television receiver or the like. In view of the circumstances, it is necessary to further increase a response speed of a liquid crystal display panel in the liquid crystal display device so that a moving picture can be finely displayed.
Against a backdrop of this, recently, it is a liquid crystal display device having an OCB (Optically Self-Compensated Birefringence type) liquid crystal display panel that has recently gained particular public attention.
(Arrangement of OCB Panel)
According to the liquid crystal display device having such an OCB liquid crystal display panel, liquid crystal molecules are provided between two substrates each of which is subjected to an alignment treatment which causes the liquid crystal molecules to be aligned in parallel to each other and in an identical direction. Wave plates are provided on respective surfaces of the two substrates. Polarizing plates are further provided on the respective surfaces of the two substrates so as to be in a crossed Nicols relationship between the polarizing plates. A negative wave plate whose main axes are hybridly aligned is used as each of the wave plates.
The following specifically describes an arrangement of a liquid crystal display device 1 having an OCB liquid crystal display panel and an orientation of liquid crystal molecules with reference to FIGS. 16 and 17. FIGS. 16 and 17 are schematic cross-sectional views each illustrating an arrangement of a liquid crystal display device 1 having the OCB liquid crystal display panel. FIG. 16 illustrates how liquid crystal molecules 52 are aligned during no voltage application, whereas FIG. 17 illustrates how the liquid crystal molecules 52 are aligned during voltage application.
A liquid crystal display panel 5 of the liquid crystal display device 1 includes a first substrate 10 and a second substrate 20 (see FIGS. 16 and 17). The first substrate 10, which serves as a TFT substrate (active matrix substrate), includes a first glass substrate 11 on which (i) a wiring layer 13 including TFT (Thin Film Transistor) elements and the like, (ii) an insulating layer 15, (iii) pixel electrodes 17, and (iv) a first alignment film 19 are provided. The second substrate 20, which serves as a counter substrate, includes a second glass substrate 21 on which a color filter 23, a counter electrode 27, and a second alignment film 29 are provided. Furthermore, a liquid crystal layer 50 including the liquid crystal molecules 52 is provided between the first substrate 10 and the second substrate 20.
Furthermore, a first optical compensation film (wave plate) 41 and a first polarizing plate 43 are provided on a surface of the first glass substrate 11 which surface is opposite to a surface on which the wiring layer 13 is provided. A second optical compensation film (wave plate) 45 and a second polarizing plate 47 are provided on a surface of the second glass substrate 21 which surface is opposite to a surface on which the color filter 23 is provided.
A backlight 70 is provided on a back side of the liquid crystal display panel 5.
More specifically, each of the first alignment film 19 and the second alignment film 29 is subjected to an alignment treatment by rubbing (a rubbing alignment treatment). As described later, the alignment treatment is carried out by rubbing the surfaces of the two substrates (the TFT substrate and the counter substrate) which surfaces face each other in an identical direction so that the liquid crystal molecules 52 have a spray orientation during no voltage application, whereas the liquid crystal molecules 52 have a bend orientation during voltage application.
Further, the polarizing plates (the first polarizing plate 43 and the second polarizing plate 47) which are attached to respective surfaces of the two substrates are arranged so that their optical axes are at respective angles of 45° and 135° (are in a crossed Nicols relationship) with respective orientation directions in which the liquid crystal molecules 52 on the surfaces of the respective substrates, that is, with a direction in which the rubbing alignment treatment is carried out.
Each of the liquid crystal molecules 52 included in the liquid crystal layer 50 generally has positive dielectric anisotropy. Note here that the liquid crystal molecule 52 having positive dielectric anisotropy refers to the one which has a characteristic in which a major axis direction of the liquid crystal molecule 52 is parallel to an electric field generated by a voltage while the voltage is being applied to the liquid crystal molecule 52.
Furthermore, the liquid crystal display device 1 includes the TFT elements for causing the liquid crystal layer 50 to be subjected to an active matrix driving. The TFT elements are provided in respective pixels, and each of the respective pixels is connected to a corresponding gate bus line and a corresponding source bus line (not illustrated) each provided on the first glass substrate.
Note that the first glass substrate 11 and the second glass substrate 21 are combined by use of sphere spacers or columnar spacers so as to be away, by a predetermined distance, from each other.
(Orientations of Liquid Crystal Molecules)
The following specifically describes orientations of the liquid crystal molecules 52 in the OCB liquid crystal display panel. The liquid crystal display device 1 having the OCB liquid crystal display panel is employed is arranged such that: the liquid crystal molecules 52 have a spray orientation during no voltage application (see FIG. 16), and the spray orientation is changed to a bend orientation during voltage application (this change is referred to as a spray-bend transition) (see FIG. 17). Then, display is carried out during the bend orientation by changing tilt angles of the respective liquid crystal molecules 52.
More specifically, right after the liquid crystal molecules 52 are filled between the first substrate 10 and the second substrate 20, the liquid crystal molecules 52 have the spray orientation (an initial orientation) in which the liquid crystal molecules 52 are substantially parallel to the first substrate 10 (see FIG. 16). Note that application of a voltage to the liquid crystal molecules 52 generally causes a transition of the liquid crystal molecules 52 from the spray orientation to the bend orientation. Namely, in a case where a relatively high voltage (e.g. 25V) is applied to the liquid crystal molecules 52 which have the spray orientation, a transition to the bend orientation occurs. The liquid crystal molecules 52 provided in a display surface have sequential transitions from spray orientation to bend orientation (see FIG. 17).
As described earlier, an actual display is carried out in a bend orientation state in the liquid crystal display device 1 having the OCB liquid crystal display panel. Therefore, it is necessary that such a spray-bend transition occur every time the liquid crystal display device 1 is turned on.
(Actual Display)
As described earlier, an actual display is carried out after a spray-bend transition has been completed, that is, in a bend orientation state. Specifically, an actual display is carried out in the following manner, for example.
Namely, in a case where an ON voltage of a voltage for normal display (a display voltage) is applied to the liquid crystal molecules 52 which is in a bend orientation state, the liquid crystal molecules 52 are caused to orient in a direction more perpendicular to the two substrates than in a case where an OFF voltage of the display voltage is applied to the liquid crystal molecules 52. In other words, the liquid crystal molecules 52 are caused to be at more right angles with the two substrates.
In any case, a white display and a black display are carried out in accordance with a change in angles of the respective liquid crystal molecules 52 in the bend orientation state.
Note that it is necessary to apply a voltage which falls within a given range to the liquid crystal molecules so as to cause the liquid crystal display device to be driven. In a case where the liquid crystal display device has an OCB liquid crystal display panel, means for initially applying an extremely high voltage (e.g. around 25V) is generally provided so that the liquid crystal molecules are subjected to a transition from spray orientation to bend orientation.
Note that the lowest voltage and the highest voltage, in a range of a display voltage obtained by removing such a high voltage, are referred to as an OFF voltage and an ON voltage, respectively. In a normally white liquid crystal display panel, a white display is carried out while the OFF voltage is being applied, whereas a black display is carried out while the ON voltage is being applied.
(Optical Compensation Film)
An optical compensation film is generally used in the liquid crystal display device 1 having the OCB liquid crystal display panel.
An object of using the optical compensation film is to obtain a greater viewing angle. Specifically, for example, Patent Literature 1 discloses a technique in which: a phase difference compensation film (an optical compensation film) corrects a phase difference caused by two planes other than an X-Z plane of a cell in which a bend orientation occurs, so as to make such a phase difference be zero (0). Note here that an X-axis, a Y-axis, and a Z-axis are defined so that an X-Y plane is a display surface of a liquid crystal display device and the Z-axis is perpendicular to the display surface.
Another object of using the optical compensation film is to improve a quality of a black display, for example, in a normally white mode. The following describes this point.
(Residual Retardation)
For example, in a case where a black display is carried out in the normally white mode, an application of the ON voltage causes the liquid crystal molecules 52 in a bulk (a region away from the two substrates) to be easy to orient perpendicularly to the two substrates. On the other hand, the liquid crystal molecules 52 in the vicinity of surfaces of the respective two substrates, specifically in the vicinity of the alignment films (the first alignment film 19 and the second alignment film 29) are difficult to orient perpendicularly to the two substrates. This is because the force is exerted on the liquid crystal molecules 52 by the alignment films.
Namely, the liquid crystal molecules 52 on the surfaces of the respective two substrates are brought into contact with the alignment films, so as to have a given pretilt angle. This causes components existing in a direction parallel to the two substrates to remain in directors of the liquid crystal molecules in the vicinity of the two substrates even while a voltage is being applied.
As a result, a retardation of the entire liquid crystal layer 50 is not completely zero even while the ON voltage is being applied to the liquid crystal layer 50. This is because a retardation remains due to the liquid crystal molecules having respective director components in the direction parallel to the two substrates (this is referred to as a residual retardation).
In a case where the residual retardation exists, light is not blocked only by the polarizing plates (the first polarizing plate 43 and the second polarizing plate 47) which are provided so as to be in the crossed Nicols relationship. This makes it impossible to satisfactorily obtain a black display.
In view of the problem, a technique of using an optical compensation film has been suggested for removing the residual retardation. For example, Patent Literature 2 discloses a method for compensating for light by inserting an optical compensation film as a method for preventing a light leakage during a black display.
According to the method in which this optical compensation film is used, a black display is realized by offsetting a retardation in a liquid crystal layer. The offsetting is realized by providing an optical compensation film between a polarizing plate and a liquid crystal display panel so that the optical compensation film has a slow axis perpendicular to a direction in which liquid crystal molecules orient. Namely, the use of the optical compensation film causes a total of a retardation of the liquid crystal layer and a retardation of a phase difference layer (the optical compensation film) to be substantially zero (0), thereby reducing a light leakage.
Note that the description is premised on a normally white mode (hereinafter referred to as a NW mode) in which a black display is carried out during high voltage application whereas a white display is carried out during low voltage application. However, the present embodiment is not limited to this. Alternatively, by changing a design of a polarizing plate and/or an optical compensation film, it is possible to realize a normally black mode (hereinafter referred to as a NB mode) in which a white display is carried out during high voltage application whereas a black display is carried out during low voltage application.
Furthermore, Patent Literature 3 discloses a technique for improving a front contrast, in which method an adjustment is carried out with respect to (i) a retardation of an optical compensation film used on a front surface side of a liquid crystal display panel and (ii) a retardation of an optical compensation film used on a rear surface side of the liquid crystal display panel.